1. Field of the Invention
The present invention relates to a level shifter irrespective of a variation of a threshold voltage, and in particular to an improved level shifter irrespective of a variation of a threshold voltage which is capable of changing input signals of 0 volt and 5 volts to the signals of -10 volts and 10 volts (these voltages can vary depending on the application), respectively, without using an additional voltage and an inverted signal of the input signal even when a threshold voltage of an NMOS transistor is higher than 5 volts.
2. Background of the Related Art
As shown in FIG. 1, the related art level shifter includes a PMOS transistor 1 and an NMOS transistor 2 which are connected in series between a power voltage Vdd and a ground voltage Vss. A PMOS transistor 3 and an NMOS transistor 4 which are connected with the PMOS transistor 1 and the NMOS transistor 2 in a latch form and are connected in series between the power voltage Vdd and the ground voltage Vss.
The gate of the PMOS transistor 1 is connected with an output terminal, and the gate of the PMOS transistor 3 is connected with the drain of the NMOS transistor 2. In addition, the gate of the NMOS transistor 2 is connected to receive an input signal Vin, and the gate of the NMOS transistor 4 is connected to receive an input signal /Vin inverted by the inverter IN.
When a 5-volt signal is inputted as the input signal Vin, the NMOS transistor 2 and the PMOS transistor 3 are turned on, and the PMOS transistor 1 and the NMOS transistor 4 are turned off. An output signal Vout of a high voltage level is outputted through the output terminal.
When a 0-volt signal is inputted as the input signal, the NMOS transistor 4 and the PMOS transistor 1 are turned on, and the NMOS transistor 2 and the PMOS transistor 3 are turned off. An output signal Vout of a low voltage level is outputted through the output terminal.
In the related art level shifter, the input signal is inverted by the inverter IN, and then the inverted signal is compared with an input signal Vin. When the threshold voltage of the NMOS transistor exceeds about 5 volts which is often the case for a polycrystalline silicon thin film transistor (Poly--Si TFT), the NMOS transistor 2 is not turned on, so that the level shifter does not operate. In order to solve this problem, additional voltage should be provided in addition to the voltage supplies of 0 volts and 5 volts. Therefore, the number of input pads of the panel is increased, and a voltage supply circuit should be additionally provided. When the threshold voltage varies greatly, which is also often the case for Poly--Si TFT, the operation speed of the above-described level shifter is influenced very much.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.